The partitioning of density‐dependent dispersal, growth and survival throughout ontogeny in a highly fecund organism

Abstract

The way in which density‐dependent effects are partitioned amongst survival, growth and dispersal are key in determining the temporal and spatial dynamics of populations. Here we propose a mechanistic approach to understanding how the relative importance of these sources of density dependence can change over ontogeny through changes in dispersal abilities, energy stores and mortality risks. Whereas the potential for active dispersal typically increases over ontogeny as a function of body size, susceptibility to starvation and predation decreases. The joint effect of these mechanisms suggests a general model for the ontogenetic sequence of how density dependence is manifested, with density dependence early in ontogeny being primarily expressed as mortality on local spatial scales, whereas later stages respond to local density in terms of dispersal and potentially growth. Here we test this model by manipulating the densities of juvenile Atlantic salmon (Salmo salarL.) at two life‐history stages in the wild. Density‐dependent mortality during the early juvenile stage (i.e. fry at onset of exogenous feeding) was accompanied by no effects on body size and weak effects on dispersal. In contrast, dispersal of older juveniles (i.e. parr 2–3 months after onset of feeding) was strongly density‐dependent, with more individuals emigrating from high‐density release sites, and with no effect of initial density on mortality. This dispersal, however, appeared insufficient to produce an ideal free distribution within the study stream, as indicated by the effect of spatial variation in density on body size by the end of the first growth season. These results demonstrate that the way density‐dependent effects are partitioned amongst survival, growth and dispersal changes throughout ontogeny. Furthermore, these changes occur in correlation with changes in individual mortality risks and dispersal abilities, and suggest a general paradigm for the way in which juvenile density‐dependence is manifest in spatially structured populations of highly fecund organisms.